The invention is a control device for controlling the exposure of film to the various lights or lamps conventionally used in continuous tone and halftone photographic processes.
Because of the complexity of such processing, based in part on the nonlinear nature of light and film sensitivities, prior art practitioners have resorted to complex electromechanical circuits and devices involving the use of many resistors and capacitors, gears, switches, cams and the like to generate logarithmic relationships; have been limited in the type of information that could be used in or entered into such an exposure computer; and have been faced with a trial and error calibration of the instrument involving the use of a gray scale over the detecting elements so that an initial relationship could be established between a "time unit" value dialed into such an instrument and the amount of real time seconds each of the various types of lamps were energized.
Thereafter the operator has been required to enter either time unit values or density values and did not have the flexibility to vary such parameters from job to job.
Some of the reasons for the complexity in halftone reproduction and processing are as follows.
A proper exposure is dependent on both the exposure time (the time the lamps are energized, variable in time units of seconds) and the relative illumination of the lamps (the intensity of the light, variable in logarithmic units of density).
Five parameters have been generally required, the shadow density of the copy, the highlight density of the copy, the screen range of the halftone screen utilized, the density range of the copy, and the excess density. Excess density is the difference between the copy density range and the halftone screen range. The copy density range is the difference, as measured on the original material to be used, between the main dot size (highlight density) and the shadow dot size (shadow density).
The main exposure is normally based on the dot size desired for the highlight density. The flash exposure, which is based on the excess density, gives the shadow dot size. Placement of a fifty percent halftone dot is controlled with the use of a technique referred to as a "bump" exposure (or a "no-screen bump" exposure) and when a bump exposure is used to vary the halftone dot, the main exposure time must be reduced.
Calibration of such an instrument also depends on the film type, chemistry, and halftone screen being used, as well as the personal preference of the operator as to the dot sizes preferred. Also, timing cycles may vary due to density shifts from variation of chemical activity levels, the emulsion speed of the film being used, or temperature shifts in the chemicals.
While microelectronics has been used in hand held camers where a limited number of variables exist, shutter speed, film speed, and light, either background light or flash exposures, prior art systems in the graphic arts field, because of the complexity of the parameters involved, have not extended microelectronics into the field of continuous tone and halftone processing in the graphic arts.